Like I already said, most likely you should add your display method to ProdRoute table.
By the way, what data do you intend to show in the display method? In your original message your method returns a field from WrkCtrResourceGroup table. But in later messages you start talking about WrkCtrActivityRequirement. Anyway, if that table has more than 1 record corresponding to one ProdRoute, how do you determine what value your display method should return? I can't answer that since I don't know your functional requirement. But you must understand that there can be multiple resource requirements for one ProdRoute record.